Emissions
- What is emitted by the plant?
- Are there any negative health affects associated with these emissions?
- Why is the stack so high?
- Would there be any visible emissions?
What is emitted by the plant?
By far the biggest combustion product is carbon dioxide, and emissions of CO2 simply equal the CO2 that is used by the wheat crop during growth. Water vapour, mainly a result of moisture in the fuel, also ends up leaving the stack in significant quantities, sometimes producing a visible plume that can be mistaken for smoke.
Other emissions would be:
- Nitrogen dioxide;
- Carbon monoxide;
- Sulphur dioxide;
- Hydrogen chloride; and
- Particulate matter.
These emissions are controlled in a variety of ways. Nitrogen dioxides, which is an inevitable product of any combustion process, is controlled by careful management of the combustion process itself (for example, staged injection of the combustion air). Carbon monoxide, which is a sign that the combustion process is incomplete, is also kept low by close attention to combustion conditions. Sulphur dioxide and hydrogen chloride are acidic gases that are produced by the reaction of the sulphur and chlorine naturally present in straw with combustion air. These acid gases are controlled by injecting a reagent such as lime into the flue gas stream which removes them. The reagent is then removed from the flue gas by a bag filtration unit which would also remove of 99% of the particulate matter produced during combustion.
The Environment Agency would set stringent limits on all of the above pollutants, which would be monitored continuously.
Are there any negative health affects associated with these emissions?
No. The environmental statement includes a detailed air quality survey which has had to demonstrate this fact. Public health is protected in the UK by means of various air quality standards which are set below the level at which any pollutant would cause any harm to human health. The environmental statement demonstrates that, even after adopting the worst possible combinations of emissions levels and weather conditions, the resulting concentration of any pollutant remain well within the required air quality standards.
Why is the stack so high?
Emissions are controlled in two ways. Firstly, the actual emissions produced by the plant must be kept to within levels set by the Environment Agency. Secondly, the emissions must be dispersed so that they are rapidly diluted to the point at which they can do no harm in the wider environment. This second control is achieved by using a high stack in combination with a buoyant flue gas stream. The stack height was calculated using Environment Agency methodology.
Would there be any visible emissions?
It has been noted above that there would sometimes be a visible plume of water vapour above the stack. The visibility of this plume is dependent on the prevailing weather conditions: in dry, warm conditions there would be no plume and in humid, cool conditions there would be.
